The present invention relates to a process for oligomerizing ethylene to light alpha olefins, mainly 1-butene, 1-hexene, 1-octene and 1-decene.
In U.S. Pat. No. 2,943,125, K. Ziegler describes a method for dimerizing ethylene into 1-butene by means of a catalyst obtained by mixing a trialkyl aluminum and a zirconium or titanium tetraalkoxide.
The oligomerization of ethylene into alpha olefins of varied molecular weights is known and involves either a stoichiometric chain growth reaction, e.g. from an organoaluminic compound, or a catalytic reaction using metals such as titanium, zirconium, chromium, nickel or rare earths, e.g. in Ziegler-type formulations.
Numerous zirconium compounds have been produced for oligomerizing ethylene into alpha olefins, often associated with various complex compounds.
Reference can e.g. be made to the use of zirconium halides combined with esters, ketones, ethers, amines, nitriles, anhydrides, chlorides of acids, amides or aldehydes described in U.S. Pat. No. 4,855,525 and WO 91 02707, or the use of the same zirconium halides combined with ligands selected from among groups of sulphur, phosphorus or nitrogen compounds, described in EP-A-241 596 and EP-A-328 728.
The products obtained with the above catalytic formulations are mainly constituted by alpha olefins having a chain length between C.sub.10 and C.sub.18. These mixtures are suitable for uses hitherto allocated to oligomers, plasticizers and detergents.
It is known that most of these catalysts lead to the formation, in addition to the desired alpha olefins, of varying quantities of high molecular weight polymers, which considerably interfere with working.